1. Field of the Invention
The present invention relates to an anti-skid brake control device for a motor vehicle.
2. Description of Related Art
As is well known, an anti-skid brake control (hereinbelow referred to as an ABS control) device is a device which is designed to prevent wheel lock even at the moment of sudden braking of a motor vehicle on a slippery road by temporarily reducing the braking pressure when the wheels are likely to lock and to stop the motor vehicle stably without losing control of the vehicle.
However, under such circumstance where the road surface conditions at the right and left sides of the motor vehicle are different, a difference between the braking forces acting on the wheels at the right side of the motor vehicle and those at the left side appears when control for maintaining an equal slip rate for the right and left side wheels of the motor vehicle is performed, with the result that a yaw moment which causes spinning of the motor vehicle around a vertical axis is generated by the braking force difference to render the motor vehicle unstable.
Under such circumstances, in a conventional ABS control device, a so-called select low control is performed in which the brake fluid pressures for wheels not likely to lock are matched to the brake pressure for a wheel likely to lock in order to improve the stability of the motor vehicle, however, such a solution results in a drawback that the braking distance of the motor vehicle increases due to the braking force decrease.
U.S. Pat. No. JP-A-1-208256(1989), for example, discloses one of the improvements ABS of the systems type above indicated in which the cornering force is increased on the one hand by reducing the target slip rate for the rear wheels under an unstable condition of a large yaw rate of the motor vehicle ,and on the other hand by reducing the target slip rate for the front wheels under a condition of a poor steering effort.
However, the above ABS control device disclosed in U.S. Pat. No. JP-A-208256 sacrifies either the braking performance or the stability of the motor vehicle and can not achieve both at the same time.
Further, even under a control which places importance on stability of a motor vehicle, no measures were taken positively to cancel out the braking force difference of the right and left wheels on a so called split .mu. road in which the road surface condition greatly differs at the right and left sides of the motor vehicle, and therefore a substantial stability of the motor vehicle could not be obtained even if the braking force difference between the right and left wheels could be reduced.